Diskarray system for suppressing disk fault

ABSTRACT

A diskarray system has a diskarray controller and a plurality of disk devices. Each of the disk devices has a media, a head, and a head position controller. The diskarray controller performs online data check operation, and stops the online data check operation over the disk device at first predetermined timing. After stopping the online data check operation, the diskarray controller issues an unload enable command to the disk device so as to move the head to a position different from positions at which the head reads or writes data from or to the media. The head position controller of the disk device moves the position of the head on the basis of the received unload enable command.

INCORPORATION BY REFERENCE

The present application claims priority from Japanese applicationJP2004-154077 filed on May 25, 2004, the content of which is herebyincorporated by reference into this application.

BACKGROUND OF THE INVENTION

The present invention relates to a storage system for storing data to beaccessed by a computer and more particular, to a diskarray system havinga plurality of disk drives or devices.

As one of types of storage systems to be connected to a computer, thereis a diskarray system. The diskarray system is also referred to as aRAID (Redundant Arrays of Inexpensive Disks) system, which is a storagesystem having a plurality of disk devices arranged in an array and acontroller (diskarray controller) for controlling the devices. In thediskarray system, a read request (data read request) and a write request(data write request) are processed at high speed through the paralleloperation of the disk, and a redundancy is added to the data. Suchdiskarray systems are classified into 5 levels according to the type ofadded redundant data and its structure, as disclosed in D. Patterson etal, “A Case for Redundant Arrays of Inexpensive Disks (RAID)”, ACMSIGMOD Conference Proceeding, June 1988, pp. 109–116.

Meanwhile, there is disclosed a technique (refer to JP-A-2002-297320)for reducing a power consumption by shifting a diskarray system to anenergy saving mode to suppress power supply to a specific disk.

SUMMARY OF THE INVENTION

Among disk devices to be applied to the diskarray system, there is adisk device which has a short operating time (life) and cannot beoperated continuously for 24 hours. Such disk devices include, forexample, a low-cost ATA (AT Attachment) disk which is designed for apersonal computer. The ATA disk is manufactured, in many cases, assumingthat its operating requirement is about 10 hours per day. Meanwhile, thediskarray system is assumed to be operated, in many cases, under itsoperating requirement of continuous 24 hours and 365 days operation.Thus, when such an ATA disk is used in the diskarray system, thereoccurs a problem that the operating time of the ATA disk per unit timeis increased and a failure rate of the disk device is correspondinglyincreased. To avoid this, a technique for suppressing the reduction ofthe reliability of the disk device used in the diskarray system orincreasing the reliability of the disk device is required.

To resolve the above problem, there is disclosed a diskarray systemwhich follows. More specifically, the diskarray system includes adiskarray controller and a plurality of disk devices for storing data.Each of the disk devices has a media on which data is to be recorded, ahead for reading and writing data from and to the media, and a headposition controller for controlling the position of the head. Thediskarray controller performs online data check to check whether or nota failure is present in a storage area of the disk device by readingdata from the storage area. That is, the diskarray controller stops theonline data check operation for the disk device, and issues an unloadvalidation (enable) command to the disk device after the stoppage of theonline data check operation in such a manner that the head of the diskdevice is moved to one position different from another position at whichthe head reads or writes data from or to the media. The head positioncontroller of the disk device functions to move the head position undercontrol of the received unload enable command.

Consequently, the reliability of the disk device used in the diskarraysystem can be increased.

Other objects, features and advantages of the invention will becomeapparent from the following description of the embodiments of theinvention taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an exemplary configuration of a computer system inaccordance with an embodiment of the present invention;

FIG. 2 shows an exemplary structure of a disk device;

FIG. 3 shown an example of a disk management table;

FIG. 4 shows an example of a processing flow of unload supportingoperations; and

FIG. 5 shows an example of a processing flow of unload monitoringoperations.

DETAILED DESCRIPTION OF THE EMBODIMENTS

An embodiment of the present invention will be explained with referenceto the accompanying drawings. The embodiment to be explained below isgiven as an example of the present invention, and the present inventionis not limited by the embodiment.

(1) Exemplary Configuration of Computer System

An exemplary configuration of a computer system in accordance with thepresent embodiment will be explained by referring to FIG. 1.

In FIG. 1, reference numeral 100 denotes a host computer, numeral 200denotes a diskarray controller, 500 denotes a disk device group, and 700denotes a management console. The diskarray controller 200 is connectedto the disk device group 500, and the diskarray controller 200 and thedisk device group 500 form a diskarray system. The host computer 100 andthe management console 700 are connected to the diskarray controller200.

The disk device group 500 is made of a plurality of disk devices 501 to504. In the diskarray system, a plurality of pieces of data stored inthe different disk devices form a parity group of the RAID. Part of thedata of the parity group is data to be accessed by the host computer100, while the other data is parity data to be generated from the datato be accessed by the host computer 100. The parity data is redundantdata which is used to restore data stored in the storage area of a diskdevice when a fault took place in the storage area of the disk device.The parity data is generated, for example, by computing an exclusive-oroperation on the other data forming the same parity group. The diskdevices 501 to 504 form an LU 520 as a logical volume.

The diskarray controller 200 has a CPU 201 for controlling the diskarraysystem, a memory 202 for storing therein a program or information to beexecuted by the CPU 201, a cache 203 for temporarily storing data to beaccessed by the host computer 100, a host FC I/F 204 for transmittingand receiving data to and from the host computer, a disk I/F 206 fortransmitting and receiving data to and from the disk device group 500, amanagement I/F 207 for transmitting and receiving control information toand from the management console 700, and a timer 208. These constituentelements of the diskarray controller are connected to each other byinternal communication lines to communicate with each other.

The memory 202 has programs, information and information storage areaswhich follow.

-   (a) A RAID control program 210 for controlling the diskarray system.-   (b) A disk command issuance program 220 for issuing a command to the    disk device group 500.-   (c) A disk management program 230 for managing the disk device group    500.-   (d) A disk management table 240 for recording information of each of    the disk devices included in the disk device group 500.-   (e) A disk operating-time measurement program 250 for measuring the    operating time of the disk device.-   (f) A disk operating-time determination program 255 for deciding the    operating time of the disk device according to a predetermined    criterion.-   (g) A disk information notification program 260 for informing the    management console 700 of information relating to the disk device.-   (h) A nonaccess-time threshold setting area 270 which is used to    decide whether or not to issue an unload command to the disk device    and which stores the threshold of the non-access time.-   (i) A timer value storage area 275 for storing a time during which    no access is made to the disk device based on a command from the    host computer 100.-   (j) A management console communication program 280 for communicating    with the management console 700.-   (k) An access monitoring program 261 for monitoring the presence or    absence of an access to the disk device.-   (l) An unload control program 262 for instructing the disk device to    enable or disable the unloading of the disk device.-   (m) An online check program 263 for performing online data check to    check whether or not data can be read from the disk device.

The online data check to be executed by the online check program 263includes, for example, two types of methods. That is, the first methodis called ‘verify’, which checks whether or not data can be read fromthe disk device. The second method is called ‘parity check’ in which aplurality of pieces of data forming a parity group are read from aplurality of disk devices to checks whether or not the parity data havea relation of data consistency therebetween as the parity group of theRAID (for example, the parity data forms an exclusive-or on the otherdata belonging to the same parity group).

The online data check is executed under control of the diskarraycontroller while the diskarray controller 200 executes an access requestissued from the host computer 100 to the diskarray system. Morespecifically, data is read from the storage area of the disk deviceunder control of the online check program 263 regardless of an accessfrom the host computer 100. In the aforementioned first method, thesystem is judged as normal when the data reading is successful, whilethe system is judged as an error when the data reading ends in afailure. In the aforementioned second method, the contents of the readdata is further checked by the online check program 263 so that thesystem is judged as normal when the data has a consistency as the dataof the parity group, whereas the system is judged as an error when thedata has no consistency thereas. When an error is detected through theonline data check, the diskarray controller 200 restores the data so farstored in the storage area where the error was detected, from databelonging to the same parity group as the so-far-stored data; and storesthe restored data in another storage area of the disk device.

The unloading operation will be explained later.

The management console 700 has an input unit 710 for accepting setinformation to be set in the diskarray controller 200 from a user or amaintenance man and an output unit 720 for outputting informationacquired from the diskarray controller 200. The input unit 710 is akeyboard, a mouse or the like; and the output unit 720 is a display orthe like.

FIG. 2 shows an exemplary structure of a disk device 501. The diskdevices 502 to 504 have a structure similar to the disk device 501.

Reference numeral 450 denotes a media to be rotated by a rotarymechanism for storing data therein, and numeral 460 denotes a head forwriting or reading data to or from the media 450. The rotary mechanismof the media 450 is not illustrated.

Numeral 434 denotes an external I/F, which forms an interface with anexternal device and in the present embodiment, is connected to the diskI/F 206 of the diskarray controller 200. Numeral 431 is a processor forcontrolling the disk device 501, 432 denotes a memory for storingtherein programs and data to be executed by the processor 431, 435denotes a head position controller for controlling the position of thehead 460, and 436 denotes a media I/F as an interface for writing orreading data to or from the media 450 via the head 460.

The memory 432 has programs and storage areas which follow.

-   (a) A command reception program 441 for receiving a command from the    diskarray controller 200 via the external I/F 434.-   (b) A data transfer program 442 for executing data transfer within    the disk device 501.-   (c) An address translation program 443 for translating an address    specified by the command received from the diskarray controller 200    into an address indicative of an actual position on the media 450.-   (d) A data buffer 444 for buffering data.-   (e) A media access program 445 for controlling access to the media    450.-   (f) A head positioning program 446 for determining the position of    the head 460.-   (g) An unload control program 451 for controlling loading or    unloading of the head 460.

Explanation will now be made as to the unloading. In the disk device ofa load/unload type, the head 460 can be retracted to an unload positionaway from the media 450. Since the unload position is away from themedia, the head cannot read or write data from or to the media and headwill not be brought into contact with the media. Accordingly, when thehead 460 is retracted to the unload position, the contact between thehead 460 and the media 450 can be avoided and the failure rate of thedisk device can be reduced. In this connection, when the head 460 is atthe unload position, the head 460 is usually supported by a head support(not shown). In the present embodiment, however, it may be assumed thatthe head is at the unload position, so long as the media 450 is notpresent under the head and the head is located in such a condition as toable to avoid any contact between the head 460 and the media 450,including the above condition that the head is supported by the support.

Even when the head 460 is retracted to the unload position, the media450 itself can continue its rotation. Thus, when there is an access tothe disk device, the head 460 can be moved in a time much shorter thanwhen the rotation of the media 450 is stopped to a load position, thatis, where the head can read or write data from or to the media 450. Whenthere is an access from the host computer 100 to data recorded on themedia 450 during stoppage of the rotation of the media, it takes a timefor the media to resume its rotation. Therefore, the access time isprolonged by a time after issuance of an access command from the hostcomputer 100 until the diskarray system responds to the command, and theresponse of the diskarray system to the host computer 100 may time out.Even when the head is retracted to the unload position, the sustainedrotation of the media 450 allows the head to be returned to a loadposition in a short time to access data recorded on the media 450. Thusthis can solve such a problem that the response of the diskarray systemtimes out.

FIG. 3 shows an example of the disk management table 240.

Parameters stored in the disk management table 240 are “disk no.”indicative of the identification number of a disk device, “cumulativeoperating time” indicative of a cumulative operating time of the diskdevice, “cumulative unload time” indicative of a cumulative unload timeof the disk device, “differential operating time” indicative of anoperating time of the disk device within a specific period,“differential unload time” indicative of an unload time within thespecific period, “cumulative operating time threshold” indicative of athreshold of the cumulative operating time, “cumulative non-unload ratethreshold” indicative of a threshold of a cumulative non-unload rate asa ratio of the cumulative not-loaded time, i.e., the cumulativeload-condition time, and “differential non-unload rate threshold”indicative of a threshold of a differential non-unload rate as a ratioof a not-loaded time, i.e., a load-condition time within the specificperiod.

In this case, the cumulative non-unload rate is defined by a percentageof ((1−‘cumulative unload time’)/‘cumulative operating time’). Thedifferential non-unload rate is defined by a percentage of((1−‘differential unload time’)/‘differential operating time’). For the‘cumulative operating time threshold’, a value indicative of the productlife of the disk as its operating time or indicative of the diskreaching its life expectancy as a rule of thumb, is set.

Using the input unit 710 of the management console 700, the user ormaintenance man enters ‘cumulative operating time threshold’,‘cumulative non-unload rate threshold’, ‘differential non-unload ratethreshold’, and ‘non-access time threshold’. The management console 700transmits information accepted from the input unit via the managementI/F 207 to the management console communication program 280 of thediskarray controller 200, and program 280 in turn sets the receivedinformation in the disk management table 240 or in the nonaccess-timethreshold setting area 270.

(2) Unload Function of Disk Device

Explanation will be made as to the unload function of the disk devices501 to 504, by referring to FIG. 2. Since the disk devices 501 to 504have the same structure, explanation will be made in connection with thedisk device 501 as a representative thereof.

An unload enable command or an unload disable command issued by theunload control program 262 of the diskarray controller 200 is receivedby the command reception program 441 via the external I/F 434, and thecommand reception program 441 in turn decides the type of the receivedcommand.

When the command reception program 441 determines that the receivedcommand is the unload enable command, the unload control program 451instructs the head position control 435 to move the head 460 to theunload position. When the command reception program 441 determines thatthe received command is the unload disable command, on the other hand,the unload control program 451 instructs the head position controller435 to move the head 460 to the load position.

In this connection, when the received command is the unload enablecommand, the unload control program 451 immediately issues an unloadinstruction to the head position controller 435, but after passage of aconstant time, the unload control program 451 may issue an instructionto the head position controller 435 to move the head 460 to the unloadposition. Further, when the received command is the unload disablecommand, the unload control program 451 immediately issues aninstruction to the head position controller 435, but at the timing thatthe command reception program 441 receives an I/O type command such as aread request or a write request from the diskarray controller 200, theunload control program 451 may issue an instruction to the head positioncontroller 435 to move the head 460 to the load position.

(3) Unload Supporting Operation of Diskarray Controller

The diskarray controller 200, in response to an access request from thehost computer 100, accesses the disk devices 501 to 504. The diskarraycontroller 200 executes the access to the disk devices and online datacheck when an access request is frequently issued from the host computer100. When the access request is not issued from the host computer 100,the diskarray controller 200 suppresses the online data check and putsthe head of the disk device in the unload state, thus enabling reductionof the failure rate of the disk device.

An example of a processing flow of unload supporting operations to beexecuted by the diskarray controller 200 will be explained in connectionwith an example of control over the disk device 501 by referring to aflowchart of FIG. 4.

First of all, the unload control program 262 issues an unload disablecommand to the disk device 501 to put the head 460 of the disk device501 in the load state (step 1001). The disk management program 230 thenstarts the online data check of the disk device 501 using the onlinecheck program 263 (step 1002).

And the access monitoring program 261 monitors the timer 208 to startthe measurement of a time during which no access from the host computer100 to data stored in the disk device 501 is present (step 1003).

The access monitoring program 261 monitors whether or not an accesscommand to the disk device was issued from the disk command issuanceprogram 220 to the disk I/F 206, that is, whether or not an access tothe disk device 501 based on a request from the host computer 100 waspresent (step 1004). When an access request command such as a readrequest or a write request was issued from the host computer 100 to thediskarray system, the diskarray controller 200 receives the accessrequest command and identifies one of the disk devices having data to beaccessed by the RAID control program 210. And the disk command issuanceprogram 220 issues an I/O type command to the identified disk device.Accordingly, the access monitoring program 261 monitors whether or notthe disk command issuance program 220 issued the I/O type command to thedisk I/F 206.

When there is no access to the disk device 501 based on a request fromthe host computer 100, control proceeds to a step 1006. When there is anaccess to the disk device 501 based on a request from the host computer100, the access monitoring program 261 clears the timer to zero (step1005) and proceeds to the step 1006.

In the step 1006, the access monitoring program 261 determines whetheror not the value of the timer which started the measurement in the step1003 reached the non-access time threshold set in the nonaccess-timethreshold setting area 270 (step 1006). When the timer value fails toreach the non-access time threshold, control returns to the step 1004.

When the timer value reached the non-access time threshold, the onlinecheck program 263 stops the online data check (step 1007), and theunload control program 262 issues an unload enable command to the diskdevice 501 to put the head 460 in the unload state (step 1008).

The access monitoring program 261 monitors the issuance of a commandfrom the disk command issuance program 220 to the disk I/F 206 and thepresence or absence of an access to the disk device 501 based on arequest from the host computer 100 (step 1009). In the absence of anaccess to the disk device 501 based on a request of the host computer100, the step 1009 is repeated.

In the presence of an access to the disk device 501 based on a requestof the host computer 100, control returns to the step 1001. When thehead 460 of the disk device 501 is put in the unload state and an accessto the disk device 501 occurs based on a request from the host computer100, the unload control program 262 issues an unload disable command toput the head 460 of the disk device 501 in the load state (step 1001).The subsequent processing is as follows.

In this case, it is assumed in the step 1004 that the disk access to thedisk device 501 is executed according to the request of the hostcomputer 100. However, the present invention is not limited to the diskaccess executed according to the request of the host computer 100, andthe access monitoring program 261 can be arranged to monitor varioustypes of commands which are issued from the disk command issuanceprogram 220 to cause access to the media 450 via the head 460.

Explanation has been made as to the processing flow of unload supportingoperations.

(4) Unload Monitoring Operation

The disk operating-time measurement program 250 counts a cumulativeoperating time of the disk devices 501 to 504 at constant intervals andsets the cumulative value in the “cumulative operating time” of the diskmanagement table 240.

The disk operating-time measurement program 250 also counts a time lapsefrom the step 1008 to the step 1001 in FIG. 4, and sets it in the“cumulative unload time” of the disk management table 240 as acumulative time during which the head 460 is put in the unload state.

The disk operating-time measurement program 250 further sets anoperating time and an unload time in a specific duration in the“differential operating time” and the “differential unload time” of thedisk management table 240 respectively. In this connection, the‘specific duration’ is received from the management console 700 of theuser or administrator via the input unit 710 and set in the diskarraycontroller 200 via the management I/F 207.

FIG. 5 shows an example of a processing flow of unload monitoringoperations based on the disk operating-time determination program 255.

The disk operating-time determination program 255 decides whether or notthe “cumulative operating time” of the disk management table 240exceeded the “cumulative operating time threshold” (step 2001). When the“cumulative operating time threshold” exceeded the “cumulative operatingtime threshold”, the disk operating-time determination program 255informs the management console 700 of the fact that the “cumulativeoperating time” exceeded its threshold using the disk informationnotification program 260 (step 2002), and control proceeds to a step2003. When the “cumulative operating time” fails to exceed itsthreshold, control goes to the step 2003.

Next, the disk operating-time determination program 255 determineswhether or not a cumulative non-unload rate exceeded the “cumulativenon-unload rate threshold” of the disk management table 240 (step 2003).When the cumulative non-unload rate exceeded its threshold, the diskoperating-time determination program 255 informs the management console700 of the fact that the cumulative non-unload rate exceeded itsthreshold using the disk information notification program 260 (step2004), and then control proceeds to a step 2005. When the cumulativenon-unload rate fails to exceed its threshold, control goes to the step2005.

The disk operating-time determination program 255 then determineswhether or not a differential non-unload rate exceeded the “differentialnon-unload rate threshold” of the disk management table 240 (step 2005).When the differential non-unload rate exceeded its threshold, the diskoperating-time determination program 255 informs the management console700 of the fact that the differential non-unload rate exceeded itsthreshold using the disk information notification program 260 (step2006) and then control returns to the step 2001. When the differentialnon-unload rate fails to exceed its threshold, control goes to the step2001.

Explanation has been made as to the processing flow of unload monitoringoperations. The management console 700, when receiving the notificationfrom the disk information notification program 260, outputs the notifiedinformation to the output unit 720.

In this connection, the disk information notification program 260 isoperated not only by the instruction of the disk operating-timedetermination program 255 but also by the instruction of the user ormaintenance man who received the information via the management console.For example, when the user or maintenance man wants to acquireinformation in the disk management table 240 of the diskarray controller200, he enters an acquisition request in the input unit 710 of themanagement console 700 to acquire the information. The disk informationnotification program 260 of the diskarray controller 200 accepts theinformation acquisition request via the input unit 710 and themanagement I/F 207, and informs the management console 700 of theinformation stored in the disk management table 240. The managementconsole 700 outputs the informed information to the output unit 720.

In accordance with the embodiment explained above, since the head of thedisk device is retracted to the unload position during the absence ofany access to the diskarray system from the host computer 100, thecontact between the media and head of the disk device can be suppressedand therefore the failure rate of the disk device can be reduced.

Further, when the head of the disk device is located at the unloadposition, the online data check is stopped. When the head is put in theload state, the online data check is executed. As a result, the onlinedata check can be efficiently carried out and the reliability of thedisk device can be increased. That is, since the unloading of the headand the online data check can be used combinedly, the reduction of thefailure rate of the disk device caused by the head unloading, thedetection of a faulty storage area by the online data check, as well asthe restoration of data stored in the detected faulty storage area canimprove the reliability of the disk device.

Even when the head of the disk device is retracted to the unloadposition, the rotation of the media can be maintained and thusdegradation (prolonged response time) of a response performance to thehost computer 100 can be suppressed.

In this connection, in place of the unload supporting operations shownin FIG. 4, the unload control program may alternately issue the unloaddisable command and the unload enable command to repeat the disabled andenabled unloading at intervals of a constant time. In this case, the“constant time” is desirably such a value as not to exceed aspecification limit of the unload disable/enable repetitive frequency ofthe head in the disk life, and desirably such a value as settable in thediskarray system by the user or maintenance man via the input unit 710of the management console. Even in this case, when the unload controlprogram 262 issues the unload disable command, the disk managementprogram 230 starts the execution of the online data check using theonline check program 263 after the issuance of the unload disablecommand, and stops the execution of the online check program before theunload control program 262 issues the unload enable command. As aresult, the adherence to the limitation specification of disk start/stoprepetition and thus the adherence to the limitation specification ofhead unload disable/enable repetition can be attained.

Further, the system may be arranged so that the user selects theexecution of the unload supporting operations shown in FIG. 4 (choice(A)) or the repetition of the unload disabling/enabling operation atintervals of a constant time by the unload control program (choice (B)).In this case, the user selects the above choice (A) or (B) for thecontrol of the unload enable/disable on the displayed selection screenof the output unit 720 of the management console 700, and enters hisselected result from the input unit 710. The entry from the user isinput to the diskarray controller 200 via the management I/F 207, sothat, according to the entry from the user, the CPU 201 selects andexecutes any of the above choice methods (A) and (B). Further, not onlythe above choices (A) and (B) but also another choice (C) of putting thehead always in the load state while not putting the head in the unloadstate is prepared. In this case, in a manner similar to the above, theuser may select any of the above choices (A), (B) and (C) and enter hisselected choice from the input unit 710 of the management console 700.When the choice (C) is selected, the diskarray controller 200 performsno head unload enabling operation.

In the foregoing embodiment, explanation has been made in connectionwith the ATA disk as an example of the disk device. From the viewpointof increasing the reliability of the disk device, however, a disk deviceother than the ATA type, for example, not only a disk device having areliability higher than the ATA disk but also an FC (fiber channel) diskhaving a reliability higher than the ATA type can be employed.

It should be further understood by those skilled in the art thatalthough the foregoing description has been made on embodiments of theinvention, the invention is not limited thereto and various changes andmodifications may be made without departing from the spirit of theinvention and the scope of the appended claims.

1. A diskarray system storing data accessed from a computer, comprising:a diskarray controller; and plural disks each adapted to store data;wherein each of said plural disks comprises a media adapted to recorddata thereon, a head adapted to read or write data from or to the media,and a head position controller adapted to control a position of thehead, wherein said diskarray controller is adapted to perform onlinedata check operations on the plural disks to check whether or not afailure is present in storage areas of the disks by reading data fromthe storage areas of the disks, wherein said diskarray controller stopsthe online data check operations over the disks at a first predeterminedtiming, wherein said diskarray controller is adapted to issue an unloadenable command to the disks so as to move the heads, in response to thestopping of the online data check operation, of the disks to positionsdifferent from positions at which the heads read or write data from orto the media, wherein said head position controller of the disks areadapted to move the position of said heads on the basis of said unloadenable command, wherein said diskarray controller is further adapted toissue an unload disable command to the disks at a second predeterminedtiming so as to move the heads of the disks to positions where the headscan read or write data from or to the media, and said diskarraycontroller, after said heads are moved thereto, is adapted to performthe online data check operations over said disks, wherein said firstpredetermined timing is when a predetermined time elapses after thediskarray system lastly receives an access request to the data stored insaid disks from the computer, and wherein said second predeterminedtiming is when the diskarray system firstly receives an access requestto data stored in said disks from the computer after the diskarraysystem stops the online data check operation over the disks.
 2. Adiskarray system according to claim 1, wherein said diskarray controlleris adapted to repetitively issue the unload disable command and anunload enable command at intervals of a predetermined time.
 3. Adiskarray system according to claim 1, wherein said plurality of disksare of an ATA type.
 4. A diskarray system according to claim 1, whereinsaid diskarray controller has a management interface which outputsinformation to a management console, wherein said diskarray controlleris adapted to measure an operating time of each of said plurality ofdisks and, when the operating time exceeds a predetermined threshold,informs said management console of the fact that the operating timeexceeded the threshold via said management interface.
 5. A diskarraysystem according to claim 1, wherein said diskarray controller has amanagement interface which outputs information to a management console,wherein said diskarray controller is adapted to measure a cumulativetime during which the heads are at the positions where the heads canread or write data from or to the media with respect to each of saidplurality of disks, and output measured results to said managementconsole via said management interface.
 6. A diskarray system accordingto claim 5, wherein said diskarray controller is adapted to measure atime during which the heads are at the positions where the heads canread or write data from or to the media with respect to each of saidplurality of disks, and output measured results to said managementconsole via said interface.
 7. A diskarray system according to claim 1,wherein the online data check operations are a parity-check operationsin which a plurality of pieces of data forming a parity group are readfrom said disks in a diskarray to check whether or not parity data ofsaid read data have consistency as said parity group of the diskarray.8. A diskarray system according to claim 1, wherein said diskarraycontroller is adapted to measure a time elapsed after the disarraycontroller lastly receives the access request to the data stored in saiddisks from the computer, and decides said first predetermined timing onthe basis of the measurement.
 9. A diskarray system according to claim1, wherein, in response to receipt of the unload enable command, thehead position controller in each of the plural disks moves the headposition to an unload position asynchronously with the timing when theunload enable command is received.